This is going to sound dismissively cynical, but it all comes down to politics. The public school system is accountable to way more entities than the Catholic school is. Teachers Unions wield a tremendous amount of power and influence in local and state government. Actually taking on such a political machine would require an incredible amount of activism - bordering on replacing the majority of elected positions in your district. This is especially difficult because most of the lower positions are seats that the average person never thinks about or votes in. But, the unions make sure their members know about these positions and endeavor to get out the vote.